1. Field of the Invention
The present invention relates generally to a method of producing a metal-oxide-semiconductor structure (MOS) device, and more particularly to such a method which is capable of improving tile reliability of a gate oxide film of the MOS device.
2. Description of the Prior Art
In a semiconductor integrated circuit (IC), each device element requires a gate oxide film which is highly reliable due to a high dielectric breakdown voltage and a small leakage current during operation. The material used in such semiconductor IC generally consists of a CZ-Si single crystal substrate (viz., a substrate fabricated from a silicon single crystal grown by Czochralsky method). However, the CZ-Si single crystal semiconductor substrate contains microscopic defects which will deteriorate the dielectric breakdown characteristics of the oxide film.
Accordingly, if a semiconductor substrate having such microscopic defects is used for fabricating an IC, the resultant IC would have problems, such as an oxidation induced stacking fault (OSF) and insufficient dielectric breakdown characteristics of the oxide film. In order to obtain a semiconductor substrate which is free from the OSF problem and which has an oxide film of excellent dielectric breakdown characteristics, there has been proposed a method in which a heat treatment is carried out in a hydrogen atmosphere (Japanese Patent Laid-open Publication Nos. 61-193456 and 62-123098). The proposed heat treatment is hereinafter referred to as "hydrogen annealing".
The proposed method in which the hydrogen annealing is used is employed for the production of a MOS device. One example of such known method is shown in FIG. 3.
According to the known method shown in FIG. 3, a polished wafer of CZ-Si single crystal is cleaned and then annealed in a hydrogen atmosphere. The annealed CZ-Si single crystal wafer is thermally oxidized in an appropriate ambient atmosphere, for example a dry oxygen atmosphere, in order to form a gate oxide film on the CZ-Si single crystal wafer. Then, a polysilicon film is deposited by low pressure chemical vapor deposition (LPCVD) method, after which phosphorus is deposited by a POCl.sub.3 method. Subsequently, a phosphorus diffusion step commonly termed "drive-in annealing" step is performed in an inert gas atmosphere, after which an electrode pattern is formed by photolithography which includes a sequence of processing steps such as photoresist coating, polysilicon etching and photoresist stripping.
The prior method described above, due to the heat-treating step required for achieving the hydrogen annealing, raises another problem such as slip (one of crystal defects) or contamination. In particular, the contamination problem is as reported in the Extended Abstracts of the 18th (1986 International) Conference on Solid State Devices and Materials, Tokyo, 1986, pp. 529-532, "Improvement of Silicon Surface Quality by H.sub.z anneal". The reported contamination level does not affect the time-zero dielectric breakdown (TZDB) strength of the oxide film. However, in subsequent research, the present inventor discovered that the contamination level is sufficient to exert a negative influence on the time-dependent dielectric breakdown (TDDB) characteristics of the oxide film.